Chromium Poisoning Mitigation Strategy in Strontium-Doped Lanthanum Manganite-Based Air Electrodes in Solid Oxide Fuel Cells

Chromium poisoning of the air electrode remains an obstacle to the long-term performance of solid oxide fuel cells (SOFCs). In Sr-doped LaMnO₃ (LSM) air electrodes, the poisoning process results in two types of deposits, chromium oxide (Cr₂O₃), and Mn, Cr spinel (MnCr₂O₄). The former forms electrochemically and the latter forms via a chemical reaction. By applying a small anodic reverse bias, Cr₂O₃ deposits can be removed because their formation is electrochemical in nature. However, MnCr₂O₄ deposits remain because their formation is chemical, rather than electrochemical, in nature. In situ chemical decomposition of the Mn, Cr spinel was investigated as an alternate removal method as thermodynamics supports its decomposition into constituent oxides below ∼540 °C in pure oxygen. The spinel decomposition process was characterized using thermogravimetric and X-ray diffraction analyses. The experimentally determined rate of spinel decomposition was undetectable (very slow) with isolated MnCr₂O₄ powders. The addition of 10 mol% gadolinia doped ceria (GDC) and silver powders significantly increased the rate of decomposition. However, the rate is limited by the diffusion of oxygen through the decomposed oxide layer. Although one strategy may be the addition of GDC and silver to the LSM air electrode to enhance spinel decomposition, the more effective mitigation strategy would be to prevent the formation of MnCr₂O₄ spinel in the first place through the removal of the reactants: Cr₂O₃ via electrochemical cleaning and mobile Mn ions in the zirconia electrolyte by incorporating a diffusion barrier layer such as GDC between the air electrode and electrolyte.